A typical adjustment device comprises a vehicle-fixed rail which has a base provided with detent means and also has at least one front part lying opposite the base, a sliding carriage displaceable in the rail, which sliding carriage is provided for an attachment of the deflection fitting, a manually releasable detent mechanism which can cooperate with the detent means and can arrest the sliding carriage in the rail, and a locking element which is functionally independent of the detent mechanism and is mounted on the sliding carriage in such a manner that it is movable, by means of an acceleration acting on the sliding carriage in a specific direction, from a position of rest in which it does not engage on the rail into a locking position in which it engages on the rail and secures the sliding carriage from further movement in the specific direction.
Such an adjustment device, as is known from U.S. patent application Ser. No. 835,455 of Apr. 8, 1997, serves to adapt the run of the safety belt optimally to the size of the respective vehicle occupant. Problems in the use of such an adjustment device can occur when the detent mechanism, in the case of an accident, is situated between two detent means constructed as detent openings, the sliding carriage therefore not being correctly secured at a detent opening by means of the detent mechanism. This problem is further intensified if the safety belt system is provided with a belt tensioner which in the case of an accident draws the belt slack out from the safety belt system. The forces thereby acting on the deflection fitting can lead to the detent mechanism not engaging reliably into the next detent opening and therefore to the sliding carriage being moved downwards relative to the vehicle, whereby the action of the belt tensioner is impaired. To prevent the sliding carriage from slipping past the next detent opening, a locking element, constructed as a locking catch and functionally independent of the detent mechanism, is mounted on the sliding carriage in such a way that it is movable, by means of an acceleration acting on the sliding carriage and directed downwards relative to the vehicle, from a position of rest in which it does not engage on the rail into a locking position in which it engages on the rail and secures the sliding carriage. The locking catch locks the sliding carriage after a movement of a few millimeters so that almost no additional belt slack is produced if the detent mechanism does not engage into a detent opening correctly.
A disadvantage in this adjustment device is that a high expenditure is necessary with regard to the dimensioning of all the components in order to reliably ensure that the locking catch reliably locks the sliding carriage under the high stresses which occur, and that it does not slip in the rail. It must in fact be ensured that the locking catch is moved towards the rail such that solely by means of the inertia forces resulting from the acceleration of the sliding carriage, the engagement edge of the locking catch digs into the rail.